The patient presented to our clinic for a second opinion. Echocardiography showed constrictive hemodynamics without evidence of pericardial effusion. A CT scan of the chest demonstrated pericardial thickening and bilateral pleural thickening, with a loculated right pleural effusion (Fig 1). Total lung capacity was 66% of the predicted normal value on pulmonary function testing. Lung entrapment was believed to be present based on the CT scan and pulmonary function testing findings, and the patient underwent a right pleural decortication. The pleural specimen revealed fibrinous pleuritis with lymphoplasmacytic inflammation (Fig 2). Immunostaining showed >30 IgG4-positive plasma cells per high-power field and an IgG4/IgG ratio of 92% (Fig 3). There were no S100-positive histiocytes or obliterative phlebitis. The pleural biopsy did not include the underlying lung parenchyma, but parenchymal involvement seemed unlikely in the absence of relevant parenchymal infiltrates on the CT scan. Serum IgG4 level was noted to be elevated to 136 mg/dL. Diagnosis of ISD was made and she was started on prednisone 40 mg/d for 2 weeks with gradual tapering over the following 2 months, by which time she was taking prednisone 7.5 mg daily as a maintenance dose. Her respiratory symptoms improved significantly during the first month, and a repeat CT scan at 3 months showed no recurrence of pleural effusion. A repeat echocardiography no longer showed signs of constrictive hemodynamics. Serum IgG4 level normalized to 71 mg/dL. Corticosteroid therapy was discontinued at 6 months, and she had no recurrence of disease at 1-year follow-up, at which time her total lung capacity had increased to 82% of the predicated normal value.